Ricin is a toxic protein which inhibits protein synthesis in eucaryotes: however, not all eucaryotes are sensitive. Rat liver ribosomes are extremely sensitive, wheat germ ribosomes are only sensitive under certain conditions and Tetrahymena ribosomes appear to be completely resistant to ricin. We can demonstrate stoichiometric binding of ricin A chain to rat liver ribosomes. Our data indicate that agents which protect ribosomes agains inactivation also prevent binding. Therefore, it appears that the enzymatic target of ricin is near or is identical to the site of A chain binding.